As a grand gesture in the dying embers of a presidency, Barack Obama’s decision to create the world’s largest marine protected area in Hawaii was a chance to flex American exceptionalism with little downside.

“I love our president,” said Kevin Chang, of conservation group Kua’aina ulu ‘auamo. Chang said Hawaiians who successfully lobbied for Obama’s extension of the Papahānaumokuākea (pronounced Pa-pa-hah-now-mo-koo-ah-keh-ah) monument are “ecstatic”.

This unabashed admiration is common in Obama’s home state. The president is routinely depicted in explosively colorful shirts on an array of products in Honolulu. He is often mentioned in revered tones. A maroon and gold fish discovered off Hawaii has been named after Obama in tribute – a worthy honor when you consider that George W Bush and his vice-president Dick Cheney have to share their names with two species of slime-mold beetles.

Papahānaumokuākea is undoubtedly a treasure – now spanning more than 580,000sq miles of the Pacific Ocean, it contains a trove of 7,000 species, including 14m seabirds, the threatened green turtle and the endangered Hawaiian monk seal.

Its creation, rushed through as one of the final touches on Obama’s legacy, surprised some in Hawaii with its speed and elicited grumbles from the fishing industry but is in keeping with the state’s embrace of kumulipo, a creation story that frames the ocean as so important that it is viewed as an extension of the land.

But the gargantuan scope of the marine reserve – it is three times the size of California – has also highlighted how little of the world’s oceans are protected. Just 4% of the total area of the oceans, about the size of Russia, is protected in any way, far less than the portion of land zoned in national parks.

Belatedly, environmentalists and governments are starting to thrash out how much of the oceans should be off limits to commercial fishing or mining, with a goal of 30% to be debated this week at the International Union for Conservation of Nature (IUCN) congress in Hawaii. Countries, particularly wealthy ones, are scrambling to catch up on ocean conservation.

If we could see what is happening in the ocean, like we can on land, we’d have taken action a long time ago. —Dr Greg Stone

“We are about 100 years behind land conservation,” said Dr Greg Stone, a marine biologist and executive vice-president of Conservation International. “We are a terrestrial species, we live on land and that’s where we look first. The ocean seems opaque and serene to us. But if we could see what is happening in the ocean, like we can on land, we’d have taken action a long time ago.”

Developing island nations that have disproportionately taken on the mantle of protection have noted the tardiness. Tommy Remengesau, president of Palau, couldn’t resist a playful jab when telling the IUCN congress about the 193,000sq mile sanctuary that surrounds the small Pacific nation.

“Congratulations, Obama. When you protect 80% of your exclusive economic zone, then you can join the big league,” Remengesau said. “How much will Palau’s efforts mean if the rest of the world isn’t with us?”

Despite its role in supporting much of the life on Earth, the ocean largely remains an unknown place used largely in short-term plunder for food or summertime paddles. Even marine scientists say the ocean, an area that covers 70% of our planet, holds stubborn mysteries.

We are not sure how long great white sharks, the best known fish in the world, live for, nor where they spend most of their time. Confusion still abounds as to what causes the glowing “milky seas” that are sighted by sailors at night. We periodically pull extraordinary new creatures from the deep, such as the colossal squid which has eyeballs that span 11 inches each.

It’s only 80 years since scientists, initially lowered into the sea in a hollow steel ball, got a proper look at what occurs in the deep sea. Even now, we have only explored a tiny fraction of the oceans.

“We have not realized what the ocean does for us,” said Dan Laffoley, marine adviser to the IUCN. “Most people’s view of the ocean is about two and a half movies – that’s the point when you pull the shades down on the plane when you fly across them.”

What we do know is that the ocean’s phytoplankton provides around 50% of the oxygen we breathe, that its water, via trans evaporation, is what we drink and that its fish provide a staple diet for half of the people on the planet. If the ocean hadn’t mopped up almost all the extra heat we are adding to the atmosphere, we’d have fried. Simply put, we wouldn’t last long without the oceans.

In return, we have looted the ocean of fish such as cod and tuna to the point they have virtually disappeared and heaped indignity upon sharks, which routinely have their fins hacked off to make soup, their bodies left to rot in the sun or simply sink to the seabed. We have riddled the waves with around five trillion pieces of plastic and changed the chemical composition of the water through our greenhouse gas emissions to the extent that coral reefs are withering and fish are literally driven mad.

Over the past year, the ocean has let out the equivalent of an anguished scream. A massive “blob” of warm water in the east Pacific saw dead whales and sea otters pile up in Alaska and exotic species such as marlin and sea snakes glide into Californian waters. A huge glacier system in Antarctica is starting to break up due to warming waters. So much coral perished on Australia’s Great Barrier Reef that divers started smelling of death.

“The oceans are so big that they seem untouchable but it’s only the last decade we’ve realized they are far more fragile than we imagined,” said Brad Ack, head of US ocean campaigning at WWF.

“We have distress signals going up all around us, with coral bleaching, the unravelling of the Arctic, tremendous pressure on fisheries, the incredible issue of carbon pollution. The oceans are failing.”

Once the problems of the oceans begin to be understood, they appear intractable. Almost everything affects the oceans, from the rubbish we throw away to the fertilizer we put on crops to the sunscreen we slather on to ward off the sun’s rays. There is no comparison on land – even the fecund Amazon isn’t as a complex an ecosystem – nor is there an easy solution.

Conservationists, therefore, hope to simplify their message with the 30% protected area goal. It won’t solve climate change, but it would reduce overfishing that has depleted a third of the world’s fisheries, put the brakes on drilling and seabed mining, and galvanize action to stem the pollution that flows from the land into the sea. It’s also a tangible target to rally around, although some would like to go further.

E.O. Wilson, the renowned conservationist, said that we should aim for “the whole shooting match” and attempt to protect almost all marine species by putting half of the world’s oceans off limits. This would be done by banning fishing on the “high seas” – the wide expanse of ungoverned ocean beyond the exclusive economic zones that end 200 miles from a nation’s coastline. Wilson has advocated a “half for them, half for us” approach for land-based conservation too.

The Alabaman waves aside any suggestion this would prove practically impossible. “I don’t want to sound like a pollyanna but my response is ‘no big problem,’” Wilson said.

“There are studies showing that if we were to prohibit all fishing in the open sea, then lo and behold the productivity of the territorial waters rises. There would be a dramatic increase in the fisheries and profits of individual countries. Do it now, do it as soon as you can. Put aside half of the world.”

Given that one in five fish that end up on the plate are caught illegally under current protections, Wilson’s optimism for a sweeping increase in no-take areas seems misplaced. But the UN is finally edging towards addressing the question of the high seas – a festering omission from the Law of the Sea treaty signed three decades ago that clarified nations’ rights along their coastlines but did little, critics contend, to conserve the oceans.

In August, Unesco put forward the idea of protecting certain prized jewels of the high sea. It imagined World Heritage status for areas such as the “white shark cafe”, the only known gathering point for white sharks in the north Pacific, the Atlantis Bank, a sunken fossil island in the Indian Ocean and the Sargasso sea, a productive area of the north Atlantic where a swirling gyre provides a concentration of algae that is vital for sharks, sea turtles and fish.

We’ve got better maps of the surface of Venus than we do of our own sea floor.

More formally, talks are taking place at the UN to thrash out a conservation plan for the high seas. Discussions are going well, according to those with knowledge of them. “There’s a lot of goodwill to this at the UN, so I’m cautiously optimistic,” said Nilufer Oral, deputy director of the Istanbul Bilgi Marine Research Center for the Law of the Sea. “I think we will have marine protected areas on the high seas, it will just depend on how it’s done.”

A more palatable goal may be to protect certain biodiversity hotspots in the high seas, such as the riot of color in coral reefs or seamounts – huge underwater volcanos that are a magnet for fish and ancient corals. Some scientists are stressing quality over quantity.

“We only know some seamounts exist because of a gravity anomaly picked up from satellites in outer space, it’s quite amazing,” said Stone. “We’ve got better maps of the surface of Venus than we do of our own sea floor.

“We need to look at areas that are intensely fished and find areas within them to give the ocean a rest. It’s tough because of resistance from the fishing industry, so we often default to areas that aren’t the most critical for protection. We need to change that.”

What keeps Stone up at night, however, is global warming. “Ocean acidification is a very scary thing,” he said. “With climate change and overfishing it’s really about picking which way we want to be poisoned to death.”

The ocean has sucked up more than 90% of the extra heat put into the atmosphere by human activity such as coal-fired power generation, transportation and agriculture. The oceans weren’t always bountiful and hospitable to life – most living things were wiped out around 200m years ago because the oceans were a toxic stew – and scientists say conditions are now shifting unfavorably for many marine creatures.

But even as we grasp how the acidifying oceans are harming shrimps and crabs, how coral nurseries are disappearing and how expanding seawater is spilling out into our cities and towns, the offshore drilling for oil, gas and minerals continues unabated. In fact it may soon expand, with a new frontier in deep sea mining set to open up over one million square kilometers of seabed.

Sally Jewell, the US interior secretary, told the IUCN congress that climate change is “the most pressing issue of our time.” And yet when asked by the Guardian whether she supports the 30% conservation goal, or a phase out of offshore drilling in federal waters, she demurred.

“The US hasn’t taken a position on that [the 30% goal],” she said. “I don’t think we have the science to know.”

Offshore extraction isn’t a free buffet. If drilling for oil goes badly it decimates wildlife and if it goes to plan it ends up warming the planet.

But Jewell said of calls to shut down fossil fuel leases in the Gulf of Mexico and the Arctic: “It’s easy to make sweeping statements but the reality is that we have an economy to run. There are jobs and businesses dependent not only on the industries in place but also affordable power.

“You can’t just flip a switch and go from fossil-fuel-based energy to non-fossil-fuel-based energy overnight. You can’t just pull the rug from under these companies.”

It’s not clear yet whether the oceans have reached a tipping point and will start to completely unravel. But even if emissions are radically cut, scientists argue that more protected areas are needed for the changes that are already in train.

“We are locking ourselves into a future where it will be hard to take our foot off the accelerator,” Laffoley said.

“What we are seeing now is running well ahead of what we can cope with. It’s really bizarre that we’ve taken so long to talk about the ocean. But we’ve got to start now.”